This disclosure relates to polycarbonate compositions and in particular to polycarbonate compositions having enhanced optical properties, methods of manufacture, and uses thereof.
Polycarbonates can provide a number of beneficial properties, and are used in the manufacture of articles and components for a wide range of applications, from automotive parts to electronic appliances. Polycarbonates, however, can undergo changes in their light absorbing and/or transmitting properties when they age, especially under the influence of heat and/or light exposure, resulting in reduced light transmission and/or color changes. Specifically, polycarbonates can develop a yellowish tint and become darker when processed into an article and when aged under the influence of heat and/or light.
There accordingly remains a need in the art for polycarbonate compositions that can provide desirable light absorbing and/or transmitting properties under various aging conditions, including polycarbonate compositions that have low yellowness and high light transmission when processed into an article (e.g. through injection molding) and when aged (e.g. at higher temperature or for a long time).
It is well known that certain additives can improve the color characteristics of virgin polycarbonate. Primary antioxidants such as hindered phenols can be added to polycarbonates to reduce discoloration of the formed articles during aging. These additives protect the material in the solid state against discoloration over time. Secondary antioxidants such as phosphites can protect the material during high temperature processing in the molten state such as with injection molding, leading to a better color of the formed article. These two types of antioxidants can be combined to optimize both the color of the article after formation as well as during its lifetime; however, this combination may not be effective to achieve desired discoloration performance in various applications such as long-pathlength lighting applications such as thick lenses, light guides etc. Although certain secondary antioxidants, e.g. those based on pentaerythriol, can be effective during molding, they reduce the longer term color stability and therefore cannot be added at high level.
Multifunctional epoxides can be used as additives for polymeric resins with the aim of increasing viscosity, improving hydrolytic stability and recyclability. However, for many polycarbonate compositions such as those having a free hydroxy content of greater than 150 ppm and/or prepared from a bisphenol having a sulfur content of greater than 2 ppm by weight, the epoxy additive can actually increase the yellowness of molded article.
EP 0 320 658 A1 indicates that a pentaerythrol based phosphite stabilizer is useful as a process stabilizer for polycarbonate compositions, but does not study the effect of (heat) aging. The patent indicates epoxy additives should not be added to the composition.
EP 0 885 929 A1 discloses compositions containing pentaerythrol based diphosphites and epoxy additives with the aim of improving processability, reducing splay and improving hydrolytic stability. No heat aging is investigated. BPA organic purity higher than 99.7% and/or restrictions to the hydroxyl content in the BPA used to make the polycarbonate are not being described.
U.S. Pat. No. 4,076,686 discloses polycarbonate compositions containing epoxy and phosphites indicating and advantage when replacing a dialkyl hydrogen phosphite partly with an epoxy additive. The advantage that is shown in color stability is small and appears to be due to the lowering of the phosphite level.
U.S. Pat. No. 7,297,381 discloses polycarbonate compositions containing epoxy and phosphite for light-diffusing films and articles. These materials contain light diffusing particles which render them unsuitable for use in lenses, light guides, display panels and other applications in which transparency is key.
US2013/0035441A1 discloses interfacially polymerized transparent polycarbonates where the polycarbonate has lower than 150 ppm hydroxyl groups and less than 2 ppm sulfur compounds with advantageous optical properties such as low starting color and good color stability in heat aging. US2013/0108820A1 discloses a process to make BPA and polycarbonate having a reduced sulfur content as well as containers made of such polycarbonate. Polycarbonates made according to these inventions exhibit improved color stability over other polycarbonates, but for long light path length applications or applications involving close proximity to light sources, further improvement is desired.
In spite of the advances described in the above references, there remains a need to achieve further improvements in starting color and color stability of polycarbonates for use in light-transmitting applications.